1. Field of the Invention
The present invention relates to a method for manufacturing an ink jet recording head, an ink jet recording head manufactured by such method, and an ink jet recording apparatus. More particularly, the invention relates to an ink jet recording head using a method whereby to create change of states of air bubbles generated in ink or the like by the application of thermal energy, and discharge ink from ink discharge ports following such change of states for the performance of recording.
2. Related Background Art
In recent years, more interest has arisen increasingly in recording by use of ink jet recording methods, because the generation of noises is small at the time of recording, which is almost negligible; recording is executable at high speeds; and also, recording is possible on an ordinary paper sheet without any particular treatment such as fixation, among other advantages.
Of these methods, an ink jet recording method disclosed in Japanese Patent Laid-Open Application No. 54-51837 and German Patent Laid-Open Publication (DOLS) No. 2,843,064, for example, has features different from those of other ink jet recording methods in that the disclosed method causes thermal energy to act upon ink to obtain active force for discharging ink droplets.
In other words, the recording method disclosed in the application or the publication referred to in the preceding paragraph is to enable thermal energy to act upon liquid (ink) so as to heat it rapidly and create air bubbles for discharging ink from ink discharge ports by means of the propagation of pressure waves in ink following the expansion and contraction of the respective air bubbles, thus enabling droplets to fly.
Particularly, the ink jet recording method disclosed in the German Patent Laid-Open Publication No. 2,843,064 has features that it is not only extremely effective when applied to a recording method of a so-called drop=on-demand type, but also, it is capable of obtaining high resolution, high quality images at high speeds, because the recording head unit used for this method is of a full-line type, which makes it easier to manufacture a highly densified multiple-orifice recording head.
FIGS. 12, 13A and 13B are views showing one example of the ink Jet recording head applicable to the recording method described above. FIG. 12 is a perspective view which shows the ink jet recording head. FIG. 13B is a plan view which shows a heater board provided with ink path walls. FIG. 13A is a cross-sectional view taken along line 13E--13E in FIG. 13B. This ink Jet recording head comprises ink discharge ports 18 each having an orifice structure arranged for discharging ink droplets; ink paths 11 conductively connected with the ink discharge ports; thermal activation units 8 provided, respectively, for the ink paths, respectively, for causing thermal energy to act upon ink; and electrothermal transducing elements. An electrothermal transducing element comprises a pair of wiring electrode layers 5a and 5b, a resistive layer 3 electrically connected with the wiring electrode layers that provide a heat generating unit 7 between the electrodes.
When ink is in contact with the heat generating unit 7 of the resistive layer 3, electric current flows through ink depending on the electrical resistive value of ink or corrosion or the like that may result from reaction between the heat generating unit of the resistive layer and ink, thus causing the resistive value of the resistive layer to change. Further, in some cases, damage or breakage may take place in this respect.
Conventionally, therefore, the resistive layer is formed by an inorganic material whose heat generating properties are excellent, such as an alloy of Ni, Cr, or the like or a metallic boride, such as ZrB.sub.2, HfB.sub.2, or the like, and then, on such resistive layer, a protection layer is arranged, which is formed by a material having a high resistance to oxidation, such as SiO.sub.2.
A method for forming an electrothermal transducing element of the kind for an ink jet recording head is generally: after the resistive layer 3 is formed on a given substrate 1, the wiring electrode layers 5a and 5b are provided, and then, the protection layers 129a, 129b, and 139 are laminated one after another. Here, there is a need for the protection layers to cover the necessary portions of the resistive layer and wiring electrode layers evenly without any defect such as pin holes in order to enable them to function sufficiently to prevent the damages that may be given to the resistive layer, the short circuit that may take place across electrodes, and the like.
However, since the wiring electrode layers 5a and 5b are formed on the resistive layer 3, steps are formed at 10 between the wiring electrode layers and the resistive layer. If such steps are covered by the protection layer, the layer thickness tends to become irregular. Therefore, the protection layer should be made thick enough to cover the steps fully so as not to cause any portions to be exposed. Here, the exposed portions are liable to take place on the step portions in particular. Thus the thickness of the protection layer should be made more than needed (more than two times the thickness of the wiring electrode layer). If the step coverage is not good enough, there is a possibility that ink is in contact with the exposed portions of the resistive layer. If such takes place, ink is electrolyzed or the resistive layer is destroyed due to reaction between ink and the heat generating unit of the resistive layer. Also, on the step portions, film quality is easily made uneven. Such unevenness in film quality may invite the local concentration of thermal stresses exerted on the protection layer due to the repeated heat generation, hence leading to the creation of cracks on the protection layer. The occurrence of such cracks allow ink to enter them to cause damages to the resistive layer. Besides, there are some cases where cracks occur on the protection layer due to pin holes or hillocks developed from the electrode material when the protection layer is formed. Conventionally, in order to solve these problems, the protection layer is made thick to improve the step coverage, thus preventing the formation of cracks and pin holes.
However, to make the protection layer thick hinders heat supply to ink, although it contributes to the enhancement of step coverage. Consequently, there are encountered problems anew as given below.
In other words, whereas heat is transferred to the protection layer through ink in the heat generating unit of the resistive layer, the so-called heat resistance between the surface of the protection layer (thermoactive portion 8) serving as the acting surface of this heat and the heat generating unit 7 of the resistive layer becomes greater if the protection layer is made thick. As a result, it is required to provide the resistive layer with an electric load more than needed. This still leads to the problems given below: (i) power saving becomes unfavorable, (ii) the excessive heat is accumulation on the substrate to make heat response inferior, and (iii) the material of the resistive layer is deteriorated (durability is lowered), among some others.
If only the protection layer is made thinner, these kinds of problems can be solved. However, it is not easy to make the protection layer thinner when forming it only by means of the conventional film formation method, such as sputtering or deposition, because the problem of durability is brought about due to the defective step coverage or the like.
With respect to recording by means of an ink jet recording head, it is generally known that the quicker ink is heated, the more is enhanced the stability of ink foaming. In other words, the shorter the pulse width of electrical signal (generally, electric pulses) that is applied to each electrothermal transducing element, the better is the foaming stability of ink. Thus, the discharging stability of flying droplets is enhanced to obtain a better recording quality. However, for the conventional ink jet recording head, the protection layer should be made thicker for the reasons described above. Therefore, the heat resistance of the protection layer becomes greater, which inevitably generates heat more than necessary. As a result, the deterioration of material (the lowered durability) ensues or the lowered heat response takes place due to the accumulation of excessive heat. Under such circumstances, therefore, it becomes difficult to make the pulse width shorter. Thus there is automatically limit to making recording quality higher after all.
Now, in order to reduce the dissipation of electric power, it is conceivable to reduce the loss of thermal energy on the wiring electrode layers by reducing the resistive value of the wiring electrode layers. More specifically, the width of an wiring electrode layer is made larger or the thickness of an wiring electrode layer is made larger, among some other methods. However, for the reasons given below, it is difficult to implement them.
(a) The width of the wiring electrode layer is confined by the arrangement density of nozzles (ink paths). For example, in a case of 300 DPI, one electrothermal transducing element should be formed in a space of 84.7 .mu.m wide. Here, if the gap between the wiring electrode layers is made narrower in this space available, the width of each wiring electrode layer can be made larger. However, since the gap between the wiring electrode layers becomes narrower, the frequency of short circuit generation is increased between the wiring electrode layers when the layers are patterned. Then, its production yield is inevitably reduced.
(b) If the wiring electrode layer is made thicker, the thickness of the protection layer should be made larger accordingly. Also, in either cases of a sputtered film and a CVD film, its formation around the step portions becomes insufficient. As a result, the protection layer is formed unevenly. Then, due to cavitation to be generated when the air bubbles vanish or due to thermal stresses generated by repeated pulses, cracks tend to occur in the vicinity of steps on the protection layer.
In order to solve these problems, there has been proposed a method (see FIG. 14A) for burying the wiring electrode layers in a groove by forming such groove on a heat accumulation layer when the heat accumulation layer 2 is provided between the substrate 1 and the resistive layer 3. (Japanese Patent Laid-Open Application No. 61-125858.) In practice, however, the patterning accuracy should deviate by approximately 0.5 to 1 .mu.m when the wiring electrode layers are patterned by means of photolithography technique or the like on such heat accumulation layer. As a result, the wiring electrode layer cannot bury the groove completely, and a gap is formed. Further, the wiring electrode layer is raised up to the outer surface of the groove to form a ridge portion as shown in FIG. 14B.